Keywords: modal pushover analysis, applicability, stiffness ratio.

When a strong earthquake load is applied to bridge structures, a nonlinear structural response accompanied by large deformation in the primary members develops and it can lead to the collapse of the structure. To ensure the safety of a structure during strong earthquake loads, an accurate estimation of the nonlinear response according to the loading history and a reasonable assessment of the seismic performance are essential. As a part of these efforts, the ATC-40 [1] and FEMA-273 [2] documents adopted a capacity-based design concept for the seismic design code which contains simplified nonlinear analysis procedures to determine the displacement demand imposed on a building expected to deform inelastically. However, previously introduced capacity-demand methods (such as the equivalent single degree of freedom) have given reasonable results to estimate the building structure behaviour under earthquake loads which are governed by the first vibration mode. Nonetheless for bridge structures such methods have a limitation because bridge structures are affected by higher modes than building structures.

In this paper, the improved modal pushover analysis (IMPA) to estimate seismic capacities of multi-span continuous bridge structures is proposed on the basis of modal pushover analysis considering all the dynamic modes of a structure. The proposed method (IMPA) eliminates the coupling effects by introducing an identical stiffness ratio, an approximate elastic deformed shape and a new distributed pushover load. Efficiency and applicability of the proposed method are verified through parametric studies for numerous models.

Accordingly, this paper also includes parametric study to verify the efficiency of the introduced IMPA through a correlation study between many analytical models such as the equivalent single degree of freedom and modal pushover analysis proposed in the previous studies via forty-eight different bridges which have different deck-pier stiffness ratios and bridge shapes.

This paper shows the maximum and average error rate through different deck-pier stiffness ratio. Also in this paper an acceptable stiffness ratio between deck and pier which makes it possible to use IMPA in predicting the seismic response of a bridge effectively is proposed on the basis of the numerical results obtained.

This paper also shows IMPA maximum and average error rates through different deck-pier stiffness ratios. In this paper, we show that regardless of the controlling methods, in low and high D regions, the analysis gives an acceptable estimation of the system behaviour. In addition, this paper shows that large error rates are caused by disagreement of the assumption that the elastic deformed shape is maintained in inelastic regions.

1

Applied Technology Council, "Seismic evaluation and retrofit of concrete buildings", Report ATC 40, November, 1996.

2

Federal Emergency Management Agency, "NEHRP guidelines for the seismic rehabilitation of buildings", FEMA 273 and "NHERP commentary on the guidelines for the seismic rehabilitation of buildings", FEMA 274. October, 1997.

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